Transferring power from an external device to an implanted device wirelessly with a Transcutaneous Energy Transfer (TET) system via, e.g., an oscillating magnetic field, generates heat, which can cause a temperature rise in the device itself and/or in any surrounding material.
Furthermore, an implanted device may require local electrical energy storage and processing when external power is interrupted or not available. This electrical energy processing and storage, typically solid-state electronics and a battery, generate heat while operating the implant, and especially when the battery is charging or discharging. For implantable medical devices, the associated temperature rise of the implant should be kept less than 2° C. to avoid tissue damage.
Many implantable medical devices, such as implanted sensors, require very little power to operate. With such low power levels (on the order of milliwatts), temperature rise within the implant is not a concern as the temperature rise easily remains below the 2° C. threshold. With higher power devices (e.g., on the order of watts and up to 15 W or more), temperature rise is a larger concern.
Electrical devices that are powered wirelessly and require more power are typically not implanted in human patients. For example, cell phones and laptops can heat up much more than 2° C. when being charged, but these temperatures are considered acceptable since they are not implantable devices.